It is well known in the metal-finishing art that metal surfaces such as aluminum, ferrous and zinc surfaces may be provided with an inorganic phosphate coating by contacting the surfaces with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves primarily as a base for the later application of a siccative organic coating composition such as paint, lacquer, varnish, primer, synthetic resin, enamel, and the like. Procedures also have been described in the art for improving the rust-resistance of metal articles by the application of a film of paint over phosphated surfaces. Although the application of a siccative coating over a phosphated metal surface improves the corrosion resistance and adhesive properties of the metal to the topcoat, there continues to be a need to improve the corrosion resistance of and siccative organic coating adhesion to metal surfaces.
The inorganic phosphate coatings generally are formed on a metal surface by means of an aqueous solution which contains phosphate ion and, optionally, certain auxiliary ions including metallic ions such as iron, sodium, manganese, zinc, cadmium, copper, lead, calcium-zinc, cobalt, nickel, and antimony ions. These aqueous solutions also may contain non-metallic ions such as halide ions, nitrate ions, sulfate ions and borate ions. Recent advances in the pretreatment field have been limited to coatings derived from solutions containing a minimum of three metal cations such as zinc, cobalt, nickel, manganese, magnesium or calcium.
Although the adhesion of siccative organic coatings to a metal surface is improved by phosphate coatings, it has been noted, for example, where ferrous metal, galvanized ferrous metal or phosphated ferrous metal parts are provided with a siccative top-coat of lacquer or enamel and such top-coat is scratched or scored during, for example, handling, forming or assembling operations, the metal substrate becomes a focal point for corrosion and for a phenomenon known as "undercutting." Undercutting, or the loosening of the top-coat in areas adjacent to a scratch or score causes a progressive flaking of the top-coat from the affected area. In severe cases, the undercutting may extend an inch or more from each side of the scratch or score, causing a loosening and subsequent flaking of the top-coat from a substantial portion, if not all, of the metal article. The undercutting also results in a reduction of the desirable corrosion-resistance properties.
The use of inorganic phosphate coatings to prevent corrosion and improve the adhesion of paints to the metal surfaces requires, as noted above, coating solutions which contain heavy metals such as nickel, zinc, chrome, manganese, magnesium, calcium, tin, cobalt, etc. Thus, it would be desirable to treat metal surfaces to improve corrosion and paint adhesion wherein the coating applied to the metal surface does not contain such heavy metals.
Solutions containing tannins have been suggested for derusting and/or producing protective coatings on steel. For example, U.S. Pat. No. 2,854,368 describes a solution for forming protective coatings on metals which comprises 1 to 8 moles of phosphoric acid and at least one tannin material in a proportion of between 1 and 35% by weight based on the weight of the solution. U.S. Pat. No. 4,944,812 describes an aqueous metal-treating solution which comprises the condensation reaction product of a vegetable tannin, an aldehyde and an amine. The solution is reported to improve the corrosion-resistance of metals which have been treated with the composition. U.S. Pat. No. 3,547,710 describes a coating composition for ferrous metal surfaces which comprises a dilute aqueous crude extract of red cedar wood containing plicatic acid and the cedar polyphenols. When such solutions are applied to ferrous metals, a coating is deposited which imparts corrosion-resistance to the ferrous metal and also enhances the adhesion between the metal surface and a paint subsequently applied thereto.
U.S. Pat. No. 3,975,214 describes a tannin containing post-treatment composition for use over zinc phosphate conversion coatings on metallic surfaces to provide an improved base for paint, lacquer, varnishes, etc. The tannin-containing solutions described in this patent are aqueous chromium-free solutions consisting essentially of a vegetable tannin in a concentration of 0.1 to 10 g/l and having a pH of less than 6, preferably between 3 and 6.